Editing Universal asynchronous receiver-transmitter (section)

==History==
Some early [[electric telegraph|telegraph]] schemes used variable-length pulses (as in [[Morse code]]) and rotating clockwork mechanisms to transmit alphabetic characters. The first serial communication devices (with fixed-length pulses) were rotating mechanical switches (''commutators''). Various [[character encoding|character codes]] using 5, 6, 7, or 8 data bits became common in teleprinters and later as computer peripherals. The teletypewriter made an excellent general-purpose I/O device for a small computer.

[[Gordon Bell]] of [[Digital Equipment Corporation|DEC]] designed the first UART, occupying an entire circuit board called a ''line unit'', for the [[Programmed Data Processor|PDP]] series of computers beginning with the [[PDP-1]].<ref name="comp-eng">C. Gordon Bell, J. Craig Mudge, John E. McNamara, [http://bitsavers.org/pdf/dec/_Books/Bell-ComputerEngineering.pdf ''Computer Engineering: A DEC View of Hardware Systems Design''], Digital Press, 12 May 2014, {{ISBN|1483221105}}, p. 73</ref><ref>{{cite web|last1=Allison|first1=David|title=Curator, Division of Information Technology and Society, National Museum of American History, Smithsonian Institution|url=http://americanhistory.si.edu/comphist/bell.htm#Inventing%20the%20UART|website=Smithsonian Institution Oral and Video Histories|access-date=14 June 2015}}</ref>
According to Bell, the main innovation of the UART was its use of [[Sampling (signal processing)|sampling]] to convert the signal into the digital domain, allowing more reliable timing than previous circuits that used analog timing devices with manually adjusted [[potentiometer]]s.<ref>''[http://archive.computerhistory.org/resources/text/Oral_History/Bell_Gordon_1/102702036.05.01.pdf Oral History of Gordon Bell]'', 2005, accessed 2015-08-19</ref> To reduce the cost of wiring, backplane and other components, these computers also pioneered [[Software flow control|flow control using XON and XOFF characters]] rather than hardware wires.

DEC condensed the line unit design into an early single-chip UART for their own use.<ref name="comp-eng" /> [[Western Digital]] developed this into the first widely available single-chip UART, the WD1402A, around 1971. This was an early example of a [[Integrated circuit|medium-scale integrated circuit]]. Another popular chip was the SCN2651 from the [[Signetics 2650]] family.

An example of an early 1980s UART was the [[National Semiconductor]] [[8250 UART|8250]], which was used in the [[IBM Personal Computer|original IBM PC]]'s Asynchronous Communications Adapter card.<ref>{{cite book |author=<!--Staff writer(s); no by-line.--> |series=Personal Computer Hardware Reference Library |title= Technical Reference 6025008 |url=http://www.minuszerodegrees.net/manuals/IBM_5150_Technical_Reference_6025005_AUG81.pdf |publisher=IBM |pages=2–123 |date=August 1981 }}</ref> In the 1990s, newer UARTs were developed with on-chip buffers. This allowed higher transmission speed without data loss and without requiring such frequent attention from the computer. For example, the popular National Semiconductor [[16550 UART|16550]] has a 16-byte [[FIFO (computing and electronics)|FIFO]], and spawned many variants, including the ''16C550, 16C650, 16C750, and 16C850''.

Depending on the manufacturer, different terms are used to identify devices that perform the UART functions. [[Intel]] called their [[Intel 8251|8251]] device a "Programmable Communication Interface" because it was actually a USART and capable of synchronous operation as well as asynchronous. It was introduced in 1979. [[MOS Technology 6551]] was known under the name "Asynchronous Communications Interface Adapter" (ACIA). The term "Serial Communications Interface" (SCI) was first used at [[Motorola]] around 1975 to refer to their start-stop asynchronous serial interface device, which others were calling a UART. Zilog manufactured a number of [[Zilog SCC|Serial Communication Controllers]] or SCCs.

Starting in the 2000s, most [[IBM PC compatible]] computers removed their external [[RS-232]] [[COM (hardware interface)|COM port]]s and used [[USB]] ports that can send data faster. For users who still need RS-232 serial ports, external [[USB adapter|USB-to-UART bridges]] are now commonly used. They combine the hardware cables and a chip to do the USB and UART conversion. [[Cypress Semiconductor]] and [[FTDI]] are two of the significant commercial suppliers of these chips.<ref>{{cite web|url=http://www.ftdichip.com/FTProducts.htm|title=FTDI Products|website=www.ftdichip.com|access-date=22 March 2018}}</ref> Although RS-232 ports are no longer available to users on the outside of most computers, many internal [[Processor (computing)|processors]] have UARTs built into their chips to give hardware designers the ability to interface with other chips or devices that use RS-232 or [[RS-485]] for communication.